The invention relates to systems for transmitting information from the bottom of a bore hole in the earth to the surface by way of pressure pulses created in a circulating mud stream in a drill string. More particularly, this invention relates to an apparatus for changing the resistance to the flow of the mud stream in the drill string to create pressure pulses therein.
The usefulness of obtaining data from the bottom of an oil, gas or geothermal well during drilling operations without interrupting these operations has been recognized for many years. However, no proven technology reliably provides this capability. Such a capability would have numerous benefits in providing for safer and less costly drilling of both exploration and production wells.
Any system that provides measurements while drilling (MWD) must have three basic capabilities: (1) to measure the down hole parameters of interest; (2) to telemeter the resulting data to a surface receiver; and (3) to receive and interpret the telemetered data.
Of these three essential capabilities, the ability to telemeter data to the surface is currently the limiting factor in the development of an MWD system.
For reasons of economy and safety it is highly desirable that the operator of a drill string be continually aware of such down hole parameters as drill bit position, temperature and bore hole pressure. Knowledge of the drill bit position during drilling would save significant time and expense during directional drilling operations. For safety it is of interest to predict the approach of high pressure zones to allow the execution of proper preventative procedures in order to avoid blowouts. In addition proper operation of the drill string requires continuous monitoring of down hole pressure. The pressure in the bore hole must be maintained high enough to keep the walls of the hole from collapsing on the drill string yet low enough to prevent fracturing of the formation around the bore hole. In addition the pressure at the bit must be sufficient to prevent the influx of gas or fluids when high pressure formations are entered by the drill bit. Failure to maintain the proper down hole pressure can and frequently does lead to loss of well control and blowouts.
Four general methods are being studied that would provide transmission of precise data from one end of the well bore to the other: mud pressure pulse, hard wire, electromagnetic waves, and acoustic methods. At this time, the mud pressure pulse method seems to be the closest to becoming commercially available.
In a typical mud pulsing system pressure pulses are produced by a mechanical valve located in a collar above the drill bit. The pulses represent coded information from down hole instrumentation. The pulses are transmitted through the mud to pressure transducers at the surface, decoded and displayed as data representing pressure, temperature, etc. from the down hole sensors. Of the four general methods named above mud pulse sensing is considered to be the most practical as it is the simplest to implement and requires no modification of existing drill pipe or equipment.
Mechanical mud pulsers known in the art are inherently slow, producing only one to five pulses per second, are subject to frequent mechanical breakdown, and are relatively expensive to manufacture and maintain. An example of such a device is U.S. Pat. No. 3,958,217 which shows a valve mechanism for producing mud pulses.